Method of refining organic acid esters containing free acids



Patented Jan. 22, 1952 METHOD OF REFINING ORGANIC ACID ESTERS CONTAINING FREE ACIDS Lester L. Weil, West Orange, N. J., and Philip Tusa, Brooklyn, N. Y., assignors to E. F. Drew & 00., Inc., New York, N. Y., a corporation of Delaware No Drawing; Application July 19, 1949, Serial No. 105,664

15 Claims. 1

The present invention is directed to fatty acid esters and more particularly to a method of refining such esters in order to purify the same and reduce the acid number thereof.

More specifically, the invention relates to the treatment of such esters as are suitable for use as plasticizers, as for example, mixed esters of rosin and higher fatty acids with such monohydric alcohols as tetrahydrofurfuryl alcohol.

It has been customary to produce esters of fatty and other organic acids and various alcohols for such purposes as plasticizers for synthetic resinous compositions, and for other uses. Such esterification does not go to completion in the ordinary commercial operation as it is not feasible to so adjust the conditions as to give complete esterification without any excess of materials. Therefore, the crude esters usually had a relatively high free fatty acid number, namely, excess of free acid, and it was customary to reduce the acid number by treatment with aqueous solutions of caustic alkali such as sodium hydroxide;

This removed much of the free acid but often some of the ester was lost in the refining operation as it tended in some measure to be emulsified with the soap formed in the refining operation. This was particularly true in the case of tall oil esters wherein, in the alkali refining, the rosin soaps have high emulsifying power, and the specific gravity of the ester is close to that of water. As a result, in some cases so much emulsification arises that refining becomes quite difficult and considerable of the ester is lost. Therefore, such an operation was not completely satisfactory, particularly as it is desirable that esters used as plasticizers be as free from such acids as possible.

The present invention is. intended and adapted to overcome the difficulties and disadvantages inherent in prior methods of refining fatty acid and similar esters. it being among the objects of the present invention to provide a method of refining which is superior to the necessity for alkali refining of the product, and which may be used in place thereof or in conjunction therewith.

It is also among the objects of the present invention to provide a method which is quite simple and which is highly effective to reduce the acid number of esters to a low point without substantial loss of product.

It is further among the objects of the present invention to provide a method of refining volatile esters which is an integral part of the method used in the manufacture of the product so that no additional equipment or steps of operation are necessary.

In the refining of such esters, it often occurs that the boiling points of the ester and excess acid are so close together that purification by fractional distillation is not only expensive but practically unfeasible. The invention is capable of removing the free acid effectively and simply,v

and without substantial loss of ester.

In practicing the present invention, there is provided a salt of an acid which is volatile at temperatures preferably below the boiling point of the ester itself. Preferably the acid is an organic acid and may include acids which are soluble in water. Among such acids, acetic acid has been found to be quite suitable. The metallic element in such salt is preferably of the alkaline earth metal group, although salts of other metals may also be used. The calcium salt has been found to be quite suitable in the operation.

The metal salt is added to the crude ester and is dissolved therein by gradually increasing the temperature above C. and usually up to 250 C. at the end of the operation. During the heating and at the high temperatures used, the calcium of the salt combines with the free acids to form salts which are non-volatile at the temperature of distillation. This releases the volatile acid, which immediately distills out of the reaction mixture as the temperature used is above I the boiling point thereof under the conditions used. Ordinaril it is preferable to place the reaction mixture under a gradually increasing vacuum as the temperature is increased so that at the end of the operation a very high vacuum is present, of the order of 3 to 10 mm. absolute pressure. The bulk of the volatile acid is condensed, and then in the continued distillation the ester is separately condensed.

At the end of the distillation operation, there ma remain a trace of acetic acid in the ester. Therefore, the product may be washed with water, whereby the acetic acid dissolves in the water and is removed from the ester. Acid numbers of less than 1.0 are readily obtained by this procedure.

In the co-pending application of Emery I.

the process of said application consists in part of a procedure as follows:

A naturally occurring mixture of rosin and unsaturated higher fatty acids including a minor proportion of saturated fatty acids and some impurities, and known as tall 011, may be the starting material. A tall oil containing, for example, about 33% of rosin is mixed in equimolecular proportions with tetrahydrofurfuryl alcohol, the amount of tall oil being 1240 grams and the amount of alcohol being about 410 grams. There is added a small amount, say .2% of lead oxide as a catalyst.

The mixture is heated for about 6 hours with a reflux condenser so as to return vaporized tetrahydrofurfuryl alcohol to the reaction mass. A mixture of the alcohol and water and of unsaponifiable matters originally present in the tall oil is allowed to distill out during the operation. A temperature of 245 to 270 is maintained during the operation, the maximum temperature being reached near the end thereof. As a result, the esterification reaction is about 82% complete.

There is then added about 40 grams of tetrahydrofurfuryl alcohol to the reaction mixture, which is heated for about '7 hours to a temperature above 275 and usually at 290 to 310 C.

The temperature is held at the maximum until the acid number has been reduced to about 4.5.

The present invention is particularly adapted for the refining of the aforesaid product. There is added thereto an amount of calcium acetate calculated to be suflicient to neutralize all of the free acid therein. There is also added 10% to excess of calcium acetate. The mixture is agitated in order to dissolve the calcium acetate. It is heated to a temperature of about 165 C. with continued agitation at atmospheric pressure. Then a vacuum is applied and the temperature is simultaneously raised to a maximum of about 200 C. During the period of heating, the amount of vacuum is increased until at the end of the operation the pressure is about 6 mm. absolute.

Distillation takes place with the ma or portion of the freed acetic acid coming over in the forerun which is collected separately. The ester itself is also distilled and it has an acid number of about 1.

An amount of water equivalent to about 10% by weight of the ester is added to the distillate and stirring is continued for a sufficient time so that any free acetic acid present in the ester is dissolved in the water. The water is removed and the purified product is recovered. It has an acid number of approximately 0.5. Instead of washing to lower the acidity, the distillate may be subjected to a steam treatment in a vacuum which results in the removal of the acetic acid with simultaneous deodorization.

Similarly, the ester may be refined by the use of barium butyrate. This salt is added in an amount about 10% in excess of that necessary to neutralize the free acid present in the ester. The procedure in the refining operation is the same as that described in connection with the use of calcium acetate for the same purpose. The

residue in the still is largely calcium rosinate and other calcium soaps.

The procedure of the present invention is quite advantageous in commercial operations. In many cases it substitutes a much simpler method of refining for the ordinary alkali refining operation. The results are much better in that a considerably lower free fatty acid number results with less difficulty. Higher yields of ester are obtained due to the absence of saponification and the avoiding of emulsification losses. The operation is quite simple and may be accomplished practically as a single operation with the manufacture of the ester itself.

In place of water washing or steam refining or in conjunction with one or both, an alkali refining to lower the acidity of the distillate may be resorted to. A dilute aqueous alkali solution is mixed with the ester resulting in the formation of, say, sodium acetate and sodium rosinate, which are readily washed out. Even though the sodium rosinate is a powerful emulsifier, the amount is so small that there is no loss of ester since there is practically no emulsification.

The process is applicable generally to the refining of esters of acids having 8 or more carbon atoms with monoor polyhydric alcohols where a low free acid content or substantially complete freedom from such acid is desirable or necessary. Esters of hydroxy acids are included. The process is particularly valuable where the boiling point of the free acid in the ester is close to that of the ester itself.

The process is applicable to other types of esters, as for example, the higher fatty acid esters of lower monohydric alcohols. As an example of the operation of the invention in connection with such esters, butyl stearate having an acid number of 10.2 was treated with 20% excess of calcium acetate necessary to completely neutralize all of the excess acidity. The mixture was heated, whereupon the calcium acetate dissolved in the ester. Heating was continued at 200 C. under a pressure of 15 mm. of mercury for a sufilcient length of time to cause the reaction to go to substantial completion. Then the temperature was lowered to -170 C. and distillation conducted under 1 mm. pressure. All of the distillate was collected in a single container including the volatile acid and the ester being refined. The condensate was washed with water at ordinary temperatures and then dried. The yield of purified butyl stearate was 92.5% of the weight of the starting material and the acid number was 0.4.

Although the invention has been described setting forth several specific embodiments thereof, many variations in the details of the procedure may be made within the spirit of the invention. For instance, in place of the alkaline earth metals, salts of other metals may be used such as of the fixed alkali metals. Other metals are also suitable, as for instance. the heavy metals which may be exemplified by lead, and others.

Other acids than those specifically named are also suitable and the acetic acid may be replaced by such acids as formic, propionic, and others. The acid should be sufiiciently volatile so it will be displaced during the distillation. It is essential in this operation that the metal salt used have an acid group rendering it soluble in the ester and it is also essential that during the heating operation the acid of said salt be volatile. The time of operation varies with the temperatures, pressures and initial free acid number specific set of conditions used in any specific case.

These and other variations in the details of operation of the invention may be made within the principles herein set forth and the invention is, therefore, to be broadly construed and not to be limited except by the character of the claims appended hereto.

We claim:

1. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being'distilled, said esters containing free organic acid whch comprises adding to such an ester a salt of an acid which is volatile below temperatures at which said ester distills, said salt being soluble in said ester, the metal of said salt being adapted to form salts of said free acid which are non-volatile at said temperatures, dissolving said salt in said ester, subjecting said mixture to distillation whereby a reaction takes place forming the salt of said free acid and freeing said volatile acid, and collecting the distilled ester.

2. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free organic acid which comprises adding to such an ester a salt of an acid which is volatile below temperatures at which said ester distills, said salt being soluble in said ester and is soluble in water, the metal of said salt being adapted to form salts of said free acid which are non-volatile at said temperature, dissolving said salt in said ester, subjecting said mixture to distillation whereby a reaction takes place forming the salt of said free acid and freeing said volatile acid, collecting the distilled ester, and washing said ester with an aqueous medium to remove free volatile acid therefrom.

3. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free organic acid which comprises adding to such an ester a salt of an acid which is volatile below temperatures at which said ester distills, said salt being soluble in said ester, the metal of said salt being adapted to form salts of said free acid which are non-volatile at said temperature, dissolving said salt in said ester, subjecting said mixture to distillation whereby a reaction takes place forming the salt of said free acid and freeing said volatile acid, collecting the distilled ester, and steaming said ester to deodorize it and to remove free volatile acid therefrom.

4. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free organic acid which comprises adding to such an ester a salt of an acid which is volatile below temperatures at which said ester distills, said salt being soluble in said ester, the metal of said salt being adapted to form salts of said free acid which are non-volatile at said temperatures, dissolving said salt in said ester, subjecting said mixture to distillation whereby a reaction takes place forming the salt of said free acid and freeing said volatile acid, collecting the distilled ester, and treating said ester with an aqueous caustic alkali to remove free acid therefrom.

5. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free organic acid which comprises adding to such an ester a salt of an acid which is volatile below temperatures at which said ester distills, said Fill 6 salt being soluble in said ester, the metal of said salt being adapted to form salts of said free acid which are non-volatile at said temperatures, dissolving said salt in said ester, subjecting said mixture to distillation whereby a reaction takes place forming the salt of said free acid and freeing said volatile acid, separately condensing said volatile acid and thereafter condensing the distilled ester.

6. A method of refining organic acid esters of acids having at least[ 8 carbon atoms and being capable of being distilled, said esters containing free acid which comprises adding to such an ester an alkali earth metal salt of a volatile organic acid, said salt being soluble in saidester, dissolving said salt in said ester, and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

7. A method of refining organic acid esters containing free acid which comprises adding to such an ester an alkali earth metal salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said ester, and subjecting said mixture to fractional distillation in a relatively high vacuum to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

8. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free acid which comprises adding to such an ester an alkali earth metal salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said ester, and subjecting said mixture to fractional distillation at temperatures above C. to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

9. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free acid which comprises adding to such an ester an alkali earth metal salt of a volatile organic acid, said salt being soluble in said ester,

dissolving said salt in said ester, and subjecting said mixture to fractional distillation at temperatures of about 150-250 C. to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

10. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free acid which comprises adding to such an ester an alkali earth metal salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said ester, and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced, and thereafter washing said ester with an aqueous medium to remove soluble substances.

11. A method of refining organic acid esters of acids having at least 8 carbon atoms and being capable of being distilled, said esters containing free acid which comprises adding to such an ester the calcium salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said ester, and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

12. A method of refining organic acid esters containing free acid which comprises adding to the mixed esters of rosin and higher fatty acids with tetrahydrofurfuryl alcohol, an alkali metal salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said esters. and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

13. A method of refining organic acid esters containing free acid which comprises adding to the mixed esters of rosin and higher fatty acids with tetrahydrofurfuryl alcohol, an alkali metal salt of acetic acid, said salt being soluble in said ester, dissolving said salt in said esters, and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

14. A method of refining organic acid esters containing free acid which comprises adding to the mixed esters of rosin and higher fatty acids with tetrahydrofurfuryl alcohol, the calcium salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said esters, and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced.

15. A method of refining organic acid esters containing free acid which comprises adding to the mixed esters of rosin and higher fatty acids with tetrahydrofurfuryl alcohol, the calcium salt of a volatile organic acid, said salt being soluble in said ester, dissolving said salt in said esters, and subjecting said mixture to fractional distillation to volatilize said volatile acid and said ester, whereby the acid number of said ester is reduced,

and thereafter washing said esters with an aqueone medium to remove residual volatile acid. LESTER L. WEIL.

PHILIP TUSA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Humphrey Aug. 10, 1937 

1. A METHOD OF REFINING ORGANIC ACID ESTERS OF ACIDS HAVING AT LEAST 8 CARBON ATOMS AND BEING CAPABLE OF BEING DISTILLED, SAID ESTERS CONTAINING FREE ORGANIC ACID WHICH COMPRISES ADDING TO SUCH AN ESTER A SALT OF AN ACID WHICH IS VOLATILE BELOW TEMPERATURES AT WHICH SAID ESTER DISTILLS, SAID SALT BEING SOLUBLE IN SAID ESTER, THE METAL OF SAID SALT BEING ADAPTED TO FORM SALTS OF SAID FREE ACID WHICH ARE NON-VOLATILE AT SAID TEMPERATURES, DISSOLVING SAID SALT IN SAID ESTER, SUBJECTING SAID MIXTURE TO DISTILLATION WHEREBY A REACTION TAKES PLACE FORMING THE SALT OF SAID FREE ACID AND FREEING SAID VOLATILE ACID, AND COLLECTING THE DISTILLED ESTER. 